Heat shock protein 90 (HSP90) inhibitors exhibit promising anti-cancer properties because various proteins associated with malignant growth, including growth factors, kinases, and hormone receptors, are dependent upon the HSP90 protein folding machinery for their maturation and/or activation. See: Bishop, S. C.; Burlison, J. A.; Blagg, B. S. J. Curr. Cancer Drug Tar. 2007, 7, 369; Donnelly, A.; Blagg, B. S. J. Curr. Med. Chem. 2008, 15, 2702; Solit, D. B.; Chiosis, G. Drug Discov. Today 2008, 13, 38; and Peterson, L. B.; Blagg, B. S. J. Future Med. Chem. 2009, 1, 267. As a molecular chaperone, HSP90 is responsible for folding these client substrates. Consequently, inhibitors of HSP90 can disrupt multiple signaling cascades simultaneously, resulting in a combinatorial attack on numerous signaling pathways. See: Xu, W.; Neckers, L. Clin. Cancer Res. 2007, 13, 1625; and Zhang, H.; Burrows, F. J. Mol. Med. 2004, 82, 488.